Significance of Residual Hydrocarbon in Sandy Soils with and without Hysteresis Effects

Abstract. Soil and groundwater contamination due to accidental spillsof hydrocarbons represents a serious environmental problem thatthreatens groundwater. The presence of such product as a nonaqueousphase liquid near the water table, or as a trapped residual inthe porous media, creates a continuous contamination source thatkeeps contributing to groundwater contamination for a very longtime. This paper gives a quick estimation of the residual hydrocarbonin porous media based on simple to collect data, including freeproduct thickness in monitoring wells and fluid and porous mediaproperties, along with the influence of hysteresis on this estimation.Such procedure can be used to come up with a quick quantitativeestimation of the components of the spill which is valuableinformation that can be used in the early stages of remediationplanning of contaminated aquifers.Theoretical estimates of the volume of the spill were made usingthe hydraulic properties of the soil and assuming a quasi-hydrostaticequilibrium distribution for the hydrocarbon and the water. Thehydrocarbon spill volume is divided into two portions: a. recoverablehydrocarbon which is the part of the hydrocarbon under positive pressurethat will flow into a monitoring well and b. residual hydrocarbon, whichis the portion of the hydrocarbon in the soil and is under negativepressure. Calculations were carried out to determine the relative amountsof the residual hydrocarbon in sandy soils. The effects of hysteresis onthe estimated residual hydrocarbon were also evaluated and found to besignificant. Similar to published experimental results on well graded anduniform sandy soils, the model predicted that a significant portion of thespill will exist as a residual phase. This portion could reach 100% of theoriginal spill for small spills, decreasing continuously as the total spillvolume increases.